Old capacitors in old radio

I suggest investigating the performance of each stage in the radio before replacing any capacitors; surprisingly, I have found that some of the grungiest wax paper capacitors, if the wax is copious and intact, have retained their characteristics more frequently than ceramic disc and other types of the same age. Some old and well-built electrolytics also are very long lived and may only need reforming if not charged recently (use a variac).

Regards,

Michael

The ones in hard plastic tubes (usually black) are paper and need to be replaced. The ones that are dipped in shiny (usually orange, or brown) epoxy are usually ok. Also, the ones that look like hollow tubes are ceramic and are usually ok. Anything else is suspect and should be tested, or replaced if there's any doubt. Andy Cuffe

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I don't understand why you think it necessary to replace the paper/wax ones without testing. In my experience those can last a _long_ time.

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It's standard procedure. They already *have* lasted a LONG time. Ask any of the experienced guys on rec.antiques.radio+phono and they'll tell you the same thing. Skimping out on replacing a 10 cent capacitor can easily burn up an expensive output tube or irreplaceable transformer. 70+ year old paper/wax capacitors are simply not reliable and can't be depended on. Do the job right and do it once, and the radio may well be playing for another

70 years.

Thanks, exactly what I needed to know!

Tin foil + waxed paper insulator, wrapped construction. I thought it was overkill to replace them all at first, but then I remembered tin whisker growth has taken out *many* far more recent AF117 transistors and that is *TIN* foil in the caps. If they aren't shorted now, they soon will be . . .

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The radio in question is about 54 years old. Post WWII.

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They'd have to be some pretty darn big whiskers. Transistors are on a different scale.

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Have you read NASA's tin whisker and their impact on satellite reliability pages?

Of specific interest is

I quote: "Whisker Length: Whiskers as long as a few millimeters are not uncommon with some experimenters observing whiskers as long as 10 mm (400 mils) in length."

And: "tin whiskers can grow through conformal coating and once exposed can then short to other tin whiskers or other exposed surfaces."

So it looks like they can be pretty darn long *and* grow through soft plastics. Wax isn't going to stop them . . .

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Water absorbtion.

Graham

TIN ? Not aluminium ?

Graham

Well I remember (from about 30+ years ago, when I was an impoverished hobbyist obtaining *nearly all* my components out of any ancient TVs, radios gramaphones etc. I found dumped + a few treasured parts I'd inherited from my Grandfather or been given by a few generous hams and techs) dismantling a few waxed paper caps that were too damaged or dirty or faulty to use. The leads were *SOLDERED* to the ends of the rolled foils . I really doubt that over 60 years ago, leads could be soldered reliably to thin *aluminium* foil.

I am not saying there weren't *any* Aluminium foil waxed paper caps with leads spot-welded to the foils, but surely the common use of Aluminium came in with the phenolic cases and phenolic impregnated dialectrics followed by early polystyrene and Mylar film caps?

Remember Tin was a lot cheaper back then and Aluminium was still pretty expensive and mostly used in the aircraft industry. I was also not favourably impressed with the durability of 40's Aluminium. All the light weight utensils I saw or handled from that period when I was a kid suffered from serious 'tin worm' with tracks of corrosion often right through the metal, yet aluminium utensils from more recent years that are now just as old are usually still servicable or have failed with localised pitting, not wriggly 'wormtracks'.

If you have any evidence to the contrary, I would be quite happy to be corrected and extend my knowlage.

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I too remember melting the wax from those old capacitors and re-soldering the connecting wire after I had broken it from rough handling. So I suspect that they were probably tin foil. I'm going back nearly 50y. I'm 65 in Feb. Soldering irons like pokers !

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Best Regards:
                     Baron.

So what? The paper used to make the capacitors was high in acid content. As the caps age, they start to adsorb moisture which breaks down the fibers and the dielectric develops leakage.

That will change the bias voltage on the grids, which cause the tubes to conduct harder. That puts a heavier load on an already overworked power transformer. A ot of those old radios were built for a lower line voltage, which causes the transformers to run hotter due to saturation of the iron laminations.

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I'm about the same age as Baron. I used to putter with electronics and got my parts from old tvs and other stuff too.

I also tore apart capacitors and still recall having several electrolytic caps explode. Particularly those VERY old oil filled ones. So of course I'd have to investigate all the pieces.

My favorites were the Mica Capacitors. I'm still wondering how they were made, and how were the wires attached? I just liked the look of them with all the colored dots on them. Listed on site below as well as pictures of them.

In an article (below), they mention the Silver Mica caps (which came out later). It says they were plated. How does one plate mica? And how does one get the right thickness of mica, knowing how it's all random pieces. But the article does not mention the make-up of the original Mica Caps.

And what the heck is Tantalum?

This site is interesting and has some info on old caps.

snipped-for-privacy@invalid.com wrote in news:fnmsrg$uup$ snipped-for-privacy@aioe.org:

I made ceramic capacitors in the late 60's and early 70's for Sprague Electric co. so I can make some guesses as to how the micas were made, or tell you how I would do it.

Mica comes in large sheets that are rather uniform in thickness. I would use a chemical reaction

0599/415.shtml to plate the mica on both sides and measure the capacitance of the large sheet. From that value, I would know what sizes to cut the sheet into in order to make many mica capacitors of 'about the desired value'.

I would then cut the sheet up into a bunch of small capacitors, solder on leads, and then use injection molding to make the case.

Of course, each finished capacitor would be tested for value and the actual value put on the case.

We did something rather similar when making ceramic capacitors. We had a target value for the batch we were making, used silk screen printing to print patterns of electrodes onto a layer of green[unfired] ceramic laid down another layer of ceramic slip, printed more electrodes, continued until we had enough 'area' to give the desired cap. We would then break up the sheet into hundreds of 'green' capacitors, 'fire' them in a kiln, abrade the edges to expose the electrodes, dip the ends in conductive ink and bake it on, solder on leads and cover coat. We would then test them for value and stamp them.

An element. A metal. Right under Niobium and Vanadium in the periodic table, between Hafnium and Tungsten. It can be used to produce a rather high value electrolytic capacitor as aluminum is still used for some electrolytic.

>

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Thanks for the info. I always wondered how they made the caps. I never thought they would be made in sheets, and thought each was made separately. I did suspect that they would be tested to insure reliability. I'm sure none of them would be 100% accurate if made in sheets like that, plus the mica would be less likely to be accurate compared to the man made ceramics. I would imagine most of the lead soldering and cutting would all be done by machine, right?

I never learned much chemistry, I just looked up Tantalum on wikipedia. I never heard of that as a metal.

Now I am thinking how they make resistors. I'll assume they are made in long strands, sort of like spaghetti and cut up. But how they attach the leads to a piece of carbon is another thing.

Thanks again, interesting information.

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The leads were cut to length and taped to a cardboard carrier. There might be 20 or so pairs of leads per carrier strip. The carrier was placed in a jig and the chips were hand loaded. A spring held the leads against the chip and the friction between chip and leads kept the chip in place while the jig was lowered through a layer of hot oil into the molten solder.

For some parts, the chips and leads were in a jig that carried them over a fountain of molten solder (wave soldering machine).

We made our resistors by silk screen printing an organo-metallic ink onto a ceramic substrate. They were then passed through a kiln, where the organic materials burned away, leaving a thin-film resistor.

Now-days, vacuum deposition is used to make most metal film resistors.

I don't know how carbon resistors are made, sorry.

An interesting feature of a resistive film is that it has a certain resistance in 'ohms per square'.

It doesn't matter WHAT size the square is. [power rating DOES depend on size, but not resistance].

If you want higher resistance, you make the resistor LONGER. If you want lower, you make it WIDER.

Learning is fun. Have fun!

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